1. Field of the Invention
This invention relates to vacuum filterbelt apparatus of the type having a continuous belt of resilient material movable by driving means along a continuous path which includes an operative path portion at which the upper face of the belt is concave. Apertures in the belt communicate at the operative path portion with a vacuum chamber beneath the belt, so that suction filtering is effected.
2. Description of the Prior Art
In the art, vacuum filters are often used for displacing liquid from slurry mixtures. A typical example is the production of H.sub.3 PO.sub.4 from a mixture of phosphate-bearing rock an sulphuric acid. The slurry produced from this reaction consists of gypsum and phosphoric acid, and the phosphoric acid is concentrated and separated from the gypsum by means of the vacuum filter. This technique is known and requires no further explanation, and is mentioned as only one possible application of vacuum filter equipment in general.
The vacuum filterbelt apparatus described above is itself an improvement over the so-called "pan-filters" which consist of a series of separate pans which are each connected to a vacuum equipment, and which are carried round a horizontal circular path like a roundabout (or merry-go-round). Successively the pans, equipped with filters, are filled, evacuated, flushed and emptied by tilting them.
Compared with panfilters, the vacuum filterbelt is an improvement because it does not require periodic tilting movements of pans and consequently runs more smoothly and can be of a lighter construction. Furthermore, access to the filter is better and the undesirable consequences of scaling within the filter are less severe. The belt is at its top surface provided with ribs and grooves. The filtercloth rests on the ribs in the horizontal operative path portion of the belt but is deflected to travel separately from the belt outside the operative path portion, so that it is easily possible to clean the filtercloth and if necessary to remove any solid incrustation from the belt.
In the known apparatus, the belt is supported in the operative path portion by stationary support plates. The belt's own weight presses it against these support plates, so that the belt is guided in a fixed path. The belt also rests on a fixed flange at the top of the vacuum chamber. Passages in the support plates open at the top surface of the support plates. These passages allow water or air to be forced between the moving belt and the support plates, to form a layer of lubricating material or air, which considerably reduces the friction between the belt and the support plates. Thus the drag force on the belt is reduced which permits the driving system and the belt to be lighter in construction. At the same time, energy required for operation of the apparatus is saved.